The present invention is directed, in general, to process control systems and, more specifically, to a system and method for providing a non-invasive diagnostic procedure for testing the operational performance of control valves.
Many process facilities (e.g., a manufacturing plant, a mineral or crude oil refinery, etc.) are managed using distributed control systems. Typical contemporary control systems include numerous modules tailored to monitor and/or control various processes of the facility. Conventional means link these modules together to produce the distributed nature of the control system. This affords increased performance and a capability to expand or reduce the control system to satisfy changing facility needs.
Industrial control systems often employ feedback controllers for controlling the operation of one or more operating units of the system such as a heater, a pump, a motor, a valve, or a similar item of equipment. In a feedback controller a command is sent to the feedback controller that represents a desired value or setpoint (xe2x80x9cSPxe2x80x9d) for a process variable (e.g., a desired pressure, a desired temperature, a desired flow rate). A feedback signal is also sent to the feedback controller that indicates the actual value of the process variable (xe2x80x9cPVxe2x80x9d) (e.g., the actual pressure, the actual temperature, the actual rate of flow). An error signal is calculated utilizing the difference between the setpoint command and the feedback signal that indicates the actual value of the process variable.
From the error signal, the feedback controller calculates a change command to change the current setting of the operational unit. For example, if the operational unit is a motor, the change command would cause the speed of the motor to change (either increase or decrease) in order to cause the actual value of the process variable to more closely approach the desired setpoint value for the process variable.
In a simple feedback controller, the change command is proportional to the error signal. In more complex feedback controllers, the change command may be a more complex function of the error signal. One common type of feedback controller is known as a Proportional, Integral, Derivative (xe2x80x9cPIDxe2x80x9d) controller. PID controllers are capable of calculating a variety of functional relationships between an error signal and a change command signal in a feedback control system.
The relationship between the error signal and the change command greatly affects the characteristics of the control system. These characteristics include (a) the xe2x80x9cresponse timexe2x80x9d of the system (i.e., how fast the operational unit responds to the new change command); (b) the xe2x80x9covershootxe2x80x9d of the system (i.e., how much the operational unit initially exceeds its new setting); and (c) the xe2x80x9cdamping ratioxe2x80x9d of the system (i.e., how long the output values of the operational unit oscillate before eventually stabilizing at the new setting).
Industrial control systems often employ numerous control valves controlled by feedback controllers. It is not unusual for a control system to employ hundreds of control valves. In the process control industry it is difficult to effectively diagnose large numbers of control values to determine which control valves are malfunctioning. The maintenance and repair of control valves is generally conducted on an xe2x80x9cad hocxe2x80x9d basis.
Control valves that malfunction significantly may be detected when the malfunction is noticed. However, there are many control valves that malfunction at a level that is not easily detectable. The malfunctions of such control valves normally go undetected. As a result, it is not unusual in the process control industry to repair control valves that are functioning within acceptable limits while failing to repair control valves that may be in serious need of repair.
In order to detect malfunctioning control valves, it is necessary to conduct thorough diagnostic tests. The prior art techniques that are currently available for diagnosing the performance of control valves employ invasive tests that require a control valve that is being tested to be taken out of operation during the test. The prior art diagnostic processes use specially designed diagnostic test equipment to manipulate the components of a control valve and monitor the performance of the components. The prior art diagnostic processes are also expensive.
Because the prior art diagnostic processes are expensive and require the control valves to be taken out of operation, only a small fraction of control valves are diagnosed in this manner.
There is therefore a need in the art for an improved system and method for providing inexpensive and non-invasive diagnostic testing of the operational performance of control valves.
A primary purpose of the present invention is to provide an improved system and method for providing inexpensive and non-invasive diagnostic testing of the operational performance of control valves.
The system of the present invention comprises a test unit that is capable of detecting malfunctions (known as xe2x80x9cvalve faultsxe2x80x9d) in a control valve while the control valve remains in operation. The test unit of the present invention comprises a data acquisition unit that measures values of the setpoint (xe2x80x9cSPxe2x80x9d) signal, the process variable (xe2x80x9cPVxe2x80x9d) signal, and the feedback controller output (xe2x80x9cOPxe2x80x9d) signal. A diagnostic test application within the test unit uses the PV and OP data to create a distinctive PV and OP data pattern for each mode of operation of the control valve. The diagnostic test application creates and records diagnostic PV and OP data patterns for 1) the control valve operating normally, and for 2) the control valve operating with each known type of valve fault. The test unit compares these recorded PV and OP data patterns to PV and OP data patterns obtained from a control valve under test. The comparison and matching of PV and OP data patterns enables the test unit to detect and identify valve faults in the control valve without taking the control valve out of operation. The system and method of the present invention may be used in any type of process control system that has control valves.
It is an object of the present invention to provide an improved system and method for diagnosing the operational performance of a control valve without taking the control valve out of operation.
It is also an object of the present invention to provide an improved system and method for creating a diagnostic data pattern that indicates the normal operation of a control valve.
It is an additional object of the present invention to provide an improved system and method for creating a data pattern that indicates when a control valve is experiencing a valve fault.
It is an object of the present invention to provide an improved system and method for comparing the data pattern of a control valve to previously recorded diagnostic data patterns of a control valve.
It is also an object of the present invention to provide an improved system and method for detecting and identifying valve faults in a control valve by comparing a data pattern of the control valve with a previously recorded data pattern of a control valve that experienced a valve fault.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.